This invention relates to methods of chemically removing ceramic-based coatings from surfaces of components, such as components exposed to the hot gas path of gas turbine engines and other turbomachinery. More particularly, this invention is directed to a method of chemically stripping a metal particle-containing ceramic coating from such components with an acidic stripping solution containing ferric chloride, nitric acid, and phosphoric acid.
The operating environment within a gas turbine engine is both thermally and chemically hostile. Significant advances in high temperature strength, creep resistance, and fatigue resistance have been achieved through the formulation of iron, nickel and cobalt-based superalloys. Nonetheless, components in the hot gas path of a gas turbine engine are often protected by one or more coatings that provide thermal and/or environmental protection. Common examples include metallic environmental coating used alone or in combination with a ceramic thermal barrier coating (TBC), which in the latter case the environmental coating is termed a bond coat for the TBC. Components protected by environmental coatings and TBC systems exhibit greater durability as well as afford the opportunity to improve efficiency by increasing the operating temperature of a gas turbine.
Metal oxides, and particularly zirconia (ZrO2) partially or fully stabilized by yttria (Y2O3), magnesia (MgO), ceria (CeO2), calcia (CaO), and/or one or more other oxides, have been widely employed as TBC materials. TBC's are typically deposited by flame spraying, air plasma spraying (APS), low pressure plasma spraying (LPPS), or a physical vapor deposition (PVD) technique such as electron beam physical vapor deposition (EBPVD), which yields a strain-tolerant columnar grain structure. TBC adhesion typically requires the use of a bond coat, for example, a diffusion coating such as a diffusion aluminide or platinum aluminide, or an overcoat alloy such as MCrAlX alloys (where M is iron, cobalt and/or nickel, and X is an active element such as yttrium or a rare earth or reactive element) or an aluminide intermetallic (e.g., beta-phase and gamma-phase nickel aluminides). The aluminum content of these bond coat materials provides for the growth of an alumina (Al2O3) scale that protects the underlying substrate from oxidation and hot corrosion and promotes chemical bonding of the TBC.
The need to remove and replace a TBC typically arises as a result of erosion or impact damage to the TBC during engine operation, or to repair certain features such as the tip length of a turbine blade. Removal of a TBC may also be necessitated during component manufacturing to address post-coating problems such as defects in the coating, handling damage, and the need to repeat noncoating-related manufacturing operations which require removal of the ceramic, e.g., electrical-discharge machining (EDM) operations. Current state-of-the-art methods for repairing components protected by TBC often result in removal of the entire TBC system, i.e., both the ceramic TBC and the bond coat, after which the bond coat and TBC must be redeposited. One such method is to use abrasives in procedures such as grit blasting, vapor honing and glass bead peening, each of which is a slow, labor-intensive process that erodes the TBC and bond coat. As such, significant efforts have been to develop nonabrasive processes for removing TBC. Examples include autoclaving processes at elevated temperatures and pressures using caustic compounds such as hydroxides, fluoride ion cleaning, and high temperature treatments with chloride. However, each of these techniques generally has the disadvantage of being slow, which significantly limits throughput and results in relatively long turnaround times. A more rapid technique for removing TBC is disclosed in commonly-assigned U.S. Pat. No. 5,614,054 to Reeves et al. as employing a halogen-containing powder or gas, preferably ammonium fluoride (NH4F). Still other treatments include the use of aqueous solutions containing an acid fluoride salt, such as ammonium bifluoride (NH4HF2) or sodium bifluoride (NaHF2), as disclosed in commonly-assigned U.S. Pat. Nos. 6,238,743, 6,379,749 and 6,758,985.
While the above treatments represent significant advancements in TBC removal, variations in composition can render certain coatings more difficult to remove than others. Examples include ceramic coatings having compositions tailored to provide a barrier to hot corrosion for components whose substrate compositions and/or operating environments render the components particularly susceptible to hot corrosion. Such is the case with certain hot section rotating hardware, including mid seals, blade retainers, and high pressure turbine (HPT) disks of high-performance gas turbine engines. Various corrosion barrier coating materials have been proposed for such applications, including those noted and discussed in commonly-assigned U.S. Patent Application Publication Nos. 2005/0138805 and 2006/0127694, whose contents regarding coating compositions and coating processes are incorporated herein by reference. As an example, both of these publications described corrosion barrier coating materials comprising binder systems that may contain phosphates and/or chromates, with the latter publication (now U.S. Pat. No. 7,314,674 to Hazel et al.) disclosing a corrosion barrier coating that may comprise a phosphate-chromate binder containing zirconium oxide (zirconia; ZrO2) and metallic particles, such as an MCrAlX composition, where M is preferably nickel or nickel and cobalt, and X is preferably yttrium or a rare earth or reactive metal. This metal particle-containing ceramic coating can be deposited by spraying a slurry directly on a substrate surface to be protected, and then curing the phosphate-chromate binder at an elevated temperature. An optional and generally nonfunctional ceramic or glassy top coat can be deposited over the hot corrosion barrier coating to achieve a generally cosmetic effect. The composition of this metal particle-containing ceramic coating is tailored to provide a protective barrier to hot corrosion and have a coefficient of thermal expansion (CTE) approximately matching that of the underlying superalloy substrate to promote its spallation resistance.
Acceptable methods for removing coatings from critical rotating hardware can be limited to the use of 500 mesh alumina as a grit blasting media. While these methods have been found capable of removing the above-noted metal particle-containing ceramic coating prior to engine service, for example, if post-coating repairs are needed, they have not been successful at removing this coating if the coated parts are being returned after engine service. Though not wishing to be held to any particular theory, oxidation products and dirt that is converted to a glass-like substance at the high temperatures within the hot gas path of a turbine engine are believed to make removal of the coating difficult. As such, further treatment methods and solutions are needed to remove this coating material.